The aim of this study was to screen the phytotoxicity of different retentates concentrated in polyphenols and extracted from olive mill wastewater (OMW), namely, nano filtration retentate (RNF) and inverse osmosis retentate (ROI). The activity of both retentates was evaluated using bioassays on dry seeds (with concentrations of 0.0, 0.1, 0.5, 1.0, 5.0, and 10.0% and compared with CaCl2 solutions to evaluate the salinity effects), on germinated seeds (with concentrations of 0.0, 5.0, and 10.0%),

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... and on the emergence of seedlings from the soil (with concentrations of 0.0, 5.0, and 10.0%). Three indicator plant species were used: Lepidium sativum L. (cress), Solanum lycopersicum L. (tomato), and Triticum turgidum subsp. durum Desf. (durum wheat). The results were expressed as the germination rate or emergence rate (GR or ER, respectively) and as the average germination time or average emergence time (AGT or AET, respectively) depending on the bioassays. Salinity showed a certain effect on the GR. Total or near-total inhibition of germination was obtained with the highest concentrations (5.0–10.0%). The dose of 1.0% of RNF and that of 0.5% of ROI caused delays in the germination of cress. The germination of tomato was delayed by RNF and ROI at concentrations of 0.5% and 1.0%. The AGT of durum wheat was not affected by RNF, but was slightly affected by ROI. The development of the seedlings was inhibited by both retentates. The results in the Petri dishes were also confirmed in pots. Retentates could be evaluated as a basis for the development of bioherbicides.